Comparison of Intraseasonal Variation of the East Asian Summer Monsoon between El Niño Developing Years and La Niña Years

Based on various reanalysis datasets during 1979-2013, we compare the intraseasonal variation of the East Asian summer monsoon (EASM) between El Niño developing years and La Niña years. It is shown that the EASM exhibits different features in the intraseasonal variation under the two situations. During the early summer of El Niño developing years, there exist weak northerly anomalies in the high latitudes, which induce negative geopotential height anomalies over East Asian and a slightly eastward retreat of the western Pacific subtropical high (WPSH). During the late summer, a cyclonic anomaly is induced in the subtropical western Pacific with enhanced convection in the central Pacific due to El Niño forcing. As a result, geopotential height reduces significantly and the WPSH tends to retreat eastward remarkably. In La Niña years, however, the warm pool convection in the summer clearly enhances due to warmer sea surface temperature in the western Pacific in the spring. Accordingly, geopotential height reduces and the WPSH tends to retreat eastward. Besides, the intraseasonal variation of the East Asian summer monsoon exhibits complicated characters in La Niña years with the strongest anomaly occurring in July and relatively weak anomaly occurring in June and August. Although the summer mean WPSH in El Niño developing years is somewhat similar to that in La Niña years, there is a significant discrepancy in the intraseasonal variation. More importantly, the physical mechanisms for the intraseasonal variation are completely different.